1. Field of the Invention
This invention relates generally to infrared thermometers, and more particularly to an optical chopper system within an infrared thermometer. A particular use of the infrared thermometer having the optical chopper is in combination with an insertion probe for measuring body temperature in a clinical setting.
2. Prior Art
In the diagnosis and treatment of diseases, measurements of a patient's body temperature is conventionally performed by the use of mercury thermometers and electronic thermometers. Mercury thermometers have the disadvantages of requiring sterilization before each use, and require a period of time for obtaining a reasonably accurate temperature reading of generally at least a minute. Electronic thermometers are generally used with a disposable cover over the temperature probe, to avoid the necessity of sterilizing the electronic thermometer probe, but an accurate reading of body temperature generally requires at least 30 seconds, since the temperature of the sanitary sheath requires this minimum period of time to equilibrate to an patient's temperature.
Infrared thermometers have become useful recently in obtaining remote temperature readings. One such device is described in U.S. Pat. No. 4,005,605 (Michael), in which a remote reading infrared thermometer in a form of a hand held probe alternately senses radiation from an inner reference area and a target, through the same optical path. An internal mirror tilts to reflect the internal readings from a portion of the thermometer to reflect the internal infrared radiation to a sensor, for comparison with the signal from the target. Another hand held infrared thermometer probe is described in U.S. Pat. No. 4,602,642 (O'Hara et al.), in which an associated chopper unit is connected or connectable to the infrared probe for making calibrations of the instrument. A more comprehensive internal calibration is described for a thermal detection system in U.S. Pat. No. 3,465,149, where a spherical chopper unit, placed behind the collimating and focusing optics of the detector, rotates about an infrared sensor area to alternately allow infrared radiation to reach the sensor from a target and from reflections from a black body surface.
It would be desirable for an infrared thermometer to include a chopper for automatically calibrating the thermometer from as much of the internally radiated error signals as possible, rather than a limited portion of the detector or a separate chopper device. Ideally then, the error signals can be substracted from the infrared readings from a target to obtain a true reading for determination of temperature. It would also be desirable to provide such an automatically calibrating internal chopper system which would cycle through calibration and temperature measurement in predetermined optimum time periods, to provide a "snapshot" reading to minimize the effect of external thermal conditions and external radiation on internal error signals. It would also be desirable to place an optical chopper system as far forward of the internal optics of the infrared thermometer as possible, in order to insure that the error signals from as much as possible of the relevant portions of the instrument are taken into account.